1. Technical Field
This invention relates generally to a television receiving apparatus and, more particularly, is directed to an apparatus for selecting, demodulating and remodulating a signal accompanying a video signal in a television broadcast.
2. Related Information
Broadcast television signals typically comprise a video signal and at least one audio signal which may be a stereo audio signal comprising left and right speaker signals. The term "broadcast" hereinafter refers to the transmission of a video signal and audio signals or other secondary information accompanying the video signal, whether originating at the source of the video signal or not. This definition would include, for example, television signals transmitted by conventional radiated propagation, by cable transmission, or by direct satellite transmission. It would also include transmissions in which a cable operator adds secondary information, in the form of audio data or otherwise, to a video signal and transmits the combination.
In general, a television signal has a video carrier and a sound carrier to transmit the picture and sound, respectively. For stereo television broadcasts, the United States television industry uses a format (arrangement of video and audio signals in a frequency spectrum) for transmitting stereo called MTS (Multichannel Television Sound), sometimes referred to as BTSC (for Broadcast Television System Committee). This format is discussed in C. Eilers, Television Multichannel Sound--The BTSC System, IEEE Transactions on Consumer Electronics, February 1983, Vol. CE-31, No. 1, incorporated herein by reference.
The MTS signal, illustrated in FIG. 1(a), transmits stereo audio as a multiplex of two signals. The sum of the left and right audio channels (L+R) has a bandwidth of 15 KHz and frequency modulates a sound carrier. This L+R signal is compatible with all existing monaural television receivers, which have audio channel low-pass filter roll-off above 15 KHz and are not affected by the remaining portions of the signal described below. A stereo difference signal (L-R), which also has an audio bandwidth of 15 KHz, exists as an amplitude-modulated double-sideband suppressed-carrier (DSB-SC) signal centered at 31.468 KHz (two times the horizontal scan frequency). A pilot subcarrier at 15.734 KHz is used at the receiver for system identification and synchronization for demodulation of the L-R suppressed-carrier signal.
A Secondary Audio Programming (SAP) channel can be transmitted simultaneously with the stereo signal in the MTS format. The SAP signal may be transmitted in a language different from that of the stereo signal (hence the term "secondary language"). Thus, for example, the stereo signal could be transmitted in English while the secondary language signal is transmitted in Spanish or German. This allows a viewer equipped with appropriate receiving apparatus to watch the same picture signal but hear a different sound signal, typically in another language. The SAP channel has an audio bandwidth of 10 KHz and exists as a frequency modulated signal centered at 5 times the horizontal scanning frequency. The choice of subcarrier frequencies as multiples of the horizontal scan frequency minimizes the potential beat and buzz problems between the video and audio signals. The beats fall at or near zero frequency and are therefore suppressed by the normal low-frequency roll-off of the audio system. A typical MTS receiver decoder illustrated in FIG. 1(b) is well known in the prior art and makes use of a matrix for combining the L+R and L-R signals to produce separate outputs for left and right stereo channels. U.S. Pat. No. 4,048,654 to Wegner also discloses the use of left and right stereo channels to transmit different languages in monaural format instead of using a separate SAP channel for this purpose.
Japan uses a variation of the MTS format called EIA-J (Electronics Industries Association--Japan). For stereo broadcasts, it contains a L+R sum signal which frequency modulates an audio carrier (as in the MTS format, so as to be compatible with existing monaural receivers), and a L-R stereo difference signal of 14 KHz bandwidth transmitted as a frequency modulated signal on an audio carrier at 31.468 KHz (two times the horizontal scan frequency). A pilot subcarrier at 55.07 KHz (three and one half times the horizontal scan rate) is amplitude modulated with an identification tone to activate decoding circuitry in stereo receivers. When broadcasting dual languages, a first language is frequency modulated onto the first audio carrier, and a second language is frequency modulated onto the second audio carrier.
The MTS format, U.S. Pat. No. 4,048,654 disclosing two languages in a stereo format, and the Japanese EIA-J format all use a single audio carrier within the television broadcast which is multiplexed with secondary signals such as a stereo difference signal and a Secondary Audio Programming (SAP) signal (i.e., carriers for the L-R and SAP signals are multiplexed onto the main sound carrier).
Instead of the MTS format, many European countries use a dual carrier audio format for television in which two separate sound carriers are frequency modulated rather than multiplexing a single sound carrier. The dual carrier format is used to support either stereo sound reception or two independent monaural sound signals (typically in two different languages) within a television broadcast as illustrated in FIG. 1(c) and FIG. 1(d). As in the MTS system, a main sound carrier SC.sub.1 is frequency modulated with the sum L+R signal in order to maintain compatibility with monaural television receivers when broadcasting in stereo. A second, independent sound carrier SC.sub.2 is frequency modulated with only the right stereo channel at a lower amplitude than the main sound carrier. Another variation of this format transmits stereo by frequency modulating the second sound carrier with a difference signal L-R instead of the right-only channel. When transmitting two languages, each sound carrier is frequency modulated with a monaural signal in a different language as shown in FIG. 1(d). One specific example of a dual carrier format is the Phase Alternating Line-B/G or PAL-B/G used in countries such as Thailand. Other countries, including Germany and South Korea, also use the dual carrier format with slight variations. A comparison of the MTS format of FIG. 1 (a) may be made with the dual carrier format of FIG. 1 (c) by noting that all the signals shown in FIG. 1 (a) are multiplexed onto a single sound carrier such as SC.sub.1 shown in FIG. 1 (c) in the MTS format.
With the dual carrier format, conventional television sets which accept only a single sound carrier for a given broadcast cannot select the second alternate language channel for the broadcast without modifications to the television receiver. Alternatively, cable television converters or other similar devices can be constructed to incorporate dual demodulators to demodulate both primary and alternate channels, but this would result in increased costs and would not allow the user to remotely control the selection of language in a fashion consistent with the cable television converter or television receiver. Moreover, there is no programmable method of altering the signal selections within a broadcast to support future changes in cable television formats, such that different variations of the signal selection process could be practiced without hardware modifications to cable television converters or television receivers. Thus, it is desirable for a cable television converter to be designed such that it could handle different television formats by simple reprogramming rather than hardware modifications.
It may be desirable for a cable operator to add a second language or other secondary channel programming to a primary language broadcast and have both sound signals transmitted simultaneously as part of a modified broadcast. Unfortunately, either the television receiver must be modified to accept this second channel, or the cable converter apparatus must be provided with two or more frequency modulation demodulators.
U.S. Pat. No. 4,486,897 to Nagai discloses an apparatus for selecting and demodulating a composite sound signal (such as MTS or EIA-J) containing both a stereo signal (left and right channels) and a monaural alternate language sound signal. The Nagai apparatus, shown in FIG. 1(e), allows selection of either a stereo output of left and right channels, or two monaural output channels, the two outputs in the latter case typically supplied in two different languages. The prior invention uses a single FM demodulator to accomplish this result.
The prior invention, however, does not allow independent selection of signals in a dual carrier system in which two frequency modulated sound carriers are transmitted. In a dual carrier system, each monaural channel must be individually selectable and demodulated independently of the other. The matrix and switching of the prior invention requires that the first signal be comprised of the sum of a stereo signal of left and right channels, so that if the monaural signal is selected, the summed left and right channels will be correctly produced at the output of the apparatus for the primary language. It does not allow either of two carriers modulated with monaural signals to be independently selected for demodulation.
Moreover, the prior invention does not provide a remodulated output which recombines a demodulated signal with the video signal suitable for the radio frequency receiving input of a television receiver. In addition, there is no low-cost method disclosed in the prior invention for tuning the demodulation element which would be economically advantageous when used in large scale production of, for example, cable converter devices. Moreover, there is no method disclosed for allowing a stereo signal to be transparently passed through to a television receiver which can accept and demodulate such a signal when used in conjunction with the prior invention, thus degrading the usefulness of the device where such a television receiver is available.